The present invention relates to sheet-fed printing machines and, more particularly to an improved stack changing device for sheet feeders of sheet-fed printing machines.
It is a known practice, in sheet feeders of sheet printing machines or other sheet-processing machines to provide arrangements for automated stack changing. These can consist of rack-type structures, so-called remaining-stack carrying devices, which are provided with thrusting and lifting drives for horizontal and vertical movement. Such so-called nonstop stack changers are suited for example during the printing of paper sheets, i.e. in machine running, to remove remainders of worked-out sheet stacks from a pallet provided, for example, with grooves and to deposit them again on a new sheet stack subsequently installed into the sheet feeder. Known devices are marked by high construction and assembly expenditures, and require special constructions of the sheet feeders. Further, devices are used in which the remaining-stack carrying device(s) have a rack engaging into the grooves of the pallet. This rack has to be removed when the remaining-stack is joined with the newly installed sheet stack to form a whole stack comprising the two stack parts. This involves high driving forces, and very severely strains the sheets of the stack lying next to the section point. Furthermore, restraining means have to be provided that prevent a shifting of the stack parts, and in so doing severely strain the stack edges. Furthermore, the operation of the sheet feeder itself is severely hampered or even rendered impossible. The sheetflow is difficult to control during the changing process, so that again and again waste sheets result.
Devices have already been developed that partly avoid some of the disadvantages described. Thus, from DE 3931710 C2 there is known a nonstop sheet feeder for rotary sheet-fed printing machines. It has a remaining-stack carrying device which is arranged underneath a conveyor table. The remaining-stack carrying device has a closed frame on which nonstop rods are arranged, which can be driven as piston rods of individual cylinders by means of a pressure medium, and which are drivable into grooves of a pallet carrying a sheet stack. The nonstop rods lie in the driven-in state on both sides of the frame and are to be removed successively from the area of the sheet feeder. Nothing is said about the sequence of operation. The bridging of the gap conditioned by the nonstop rods between main stack and remaining-stack constitutes, in the stack unification, an obstacle to a faultless continuous process. The device is not usable, since the sheet stack must be tilted.
From DE 4203500 A1 a sheet feeder is known. It has parallel to the sheet feeder and allocated to the sheet feeder on the face side, an auxiliary stack carrying device as an independent component. With this device, over a common drive, there are provided individually drivable pointed bars which are drivable into grooves of a pallet carrying a sheet stack. The drive has individual chain drives which are coupleable onto the respective pointed bars. For guiding and accessing the chain drives, special constructive measures are required. The chain drives completely block the space in front of the sheet feeder, so that the latter is not accessible. In the stack changing, the pointed bars are removed from the stack zone, in the joining of main stack and remaining-stack, first on the outside, then in the middle, and last in the zone between the already pulled pointed bars, so that there is supposed to result a gentle depositing of the remaining-stack on the sheet stack. This, however, is possible with the requisite precision only in the case of heavy materials, such as sheets of metal, since the sheets arch in different directions and must descend over a large gap that is formed by the pointed bars.
Further, from DAS 1095297 a sheet carrier with several stack-lifting mechanisms is known. It has a fork-shaped remaining-stack carrying device, which is provided with remaining-stack bars slidable into grooves of a pallet. The device makes possible the taking over of a remainder of a sheet stack from the pallet for the continuous in-feed of the sheets while a new sheet stack is installed into the sheet feeder. The remaining-stack is connected with a separate lifting mechanism parallel to the main stack lifting mechanism inside the sheet feeder, so that the remaining-stack is continuously liftable. The operating range of the remaining-stack carrying device is restricted. The remaining-stack carrying device hampers access of the sheet feeder.
In view of the foregoing, an object of the present invention is to provide an improved stack changing device which overcomes the problems associated with prior art designs.
A more specific object of the present invention is to provide a stack changing device which can be easily integrated into a sheet feeder.
The present invention provides these and other advantages and overcomes the drawbacks of the prior art by providing a stack changing device which utilizes an improved apparatus for receiving a remaining-stack and transferring the remaining stack to a newly fed-in sheet stack.
It is advantageous that carrying bars movable independently from one another are provided in the device, which for the unburdening of the sheet material are pulled out of the stacking zone not simultaneously, but offset from one another. The drive forces are thereby kept low and the cost of the drives of the remaining-stack bars is reduced. The device can be used, therefore, instead of a manual nonstop arrangement on a sheet feeder, and also be reequipped without the need for expensive constructive measures and large space requirements.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of preferred exemplary embodiments of the invention and upon reference to the drawings wherein: